CMOS imagers have been increasingly used as low cost imaging devices. A filly compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits is beneficial in many digital imaging applications such as, for example, cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems and star trackers, among many others.
In a conventional (4T) CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge at a storage node; (3) transfer of charge from the node to an output transistor accompanied by charge amplification; (4) resetting the storage node to a known state before accumulation of image charge and selective output of reset and pixel image signals. A transfer transistor may also be used to transfer charge from a photo conversion element to the output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p−n junction photodiode or a field induced depletion region beneath a photogate.
Exemplary CMOS imaging circuits as well as detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,204,524 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al., and U.S. Pat. No. 6,326,652 to Rhodes, the disclosures of which are incorporated herein by reference
CMOS imagers typically suffer from poor signal-to-noise ratios and poor dynamic range as a result of the inability to fully store and utilize the electric charge collected by the photosensitive area. Consequently, storage capacitors have been proposed for use in connection with the light sensitive node of a CMOS pixel sensor cells to improve collected charge storage. For example, U.S. Pat. No. 6,204,524 to Rhodes describes in detail the formation of planar and trench storage capacitors electrically connected in parallel with the light sensitive node of a CMOS pixel sensor cell.
Storage capacitors may also provide useful results when electrically connected to other light sensitive and/or electrical elements of the pixel sensor cell, such as transistor gates or floating diffusion regions, for example, to affect the operation and characteristics of such various light sensitive and/or electrical elements. Capacitors connected to such various light sensitive and/or electrical elements of the pixel sensor cell help amplify the signal of an imager transistor, increase the storage capacitance of a photosite, or provide a low noise decoupling capacitor. Such storage capacitors are described in co-pending U.S. patent application Ser. No. 10/303,896, filed Nov. 26, 2002, by Howard E. Rhodes and Jeff McKee, entitled “CMOS IMAGER PIXEL DESIGNS”, the entire disclosure of which is incorporated herein by reference. While the use of storage capacitors at various locations within a pixel improve pixel operation, those capacitors have the same capacitance value for each pixel color and are not optimized for the photon to charge conversion characteristics of each color pixel.